Mr Barnes described the find as a “miracle” after he spotted the stone in among the rocks on the shore.
The faint carving, “pecked” into a slab of sandstone, is difficult to see in direct light, but features two concentric circles – or possibly one concentric circle with a spiral surrounded by an oval. The designs are “classic megalithic art”, examples of which have been found elsewhere in Britain, Ireland and Europe. Locally, the design has parallels in the Pierowall Stone from Westray and the Eday Manse stone.
Although the exact meaning of the designs is unknown, it is generally thought they were sacred or religious symbols.
County archaeologist Julie Gibson added: “Although there are no structures visible near the spot where the stone was found, it seems likely that the stone may have formed part of a chambered cairn, or other structure, which was once below the eroding storm beach.”
The stone was removed from the beach and transferred to the Orkney Museum.